For over twenty years, much attention has been given to passenger-carrying ground vehicles, capable of traveling at high speed, e.g. 100 to 300 miles per hour (160 to 480 km/hour). There is a significant need for such transportation between neighboring cities of high population density where automobile and air transport are not efficient, such as in the northeast corridor of the United States. The United States also has need for such transportation over longer distances and in areas of more diffuse population.
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Magnetic levitation ("Maglev") is any system where a vehicle is suspended (and usually guided) by electromagnetic forces, rather than by wheels or aerodynamic forces. Electromagnetic forces typically provide propulsive force also. It has been suggested that Maglev travel would be as fast or faster than airplanes for distances of less than 600 miles (1000 km.).
Maglev has advantages over other modes of travel. It is powered by electricity, which, in the United States, is only 30% dependent on petroleum. Further, it would use only a quarter to a half as much energy per passenger-mile traveled as short haul jet aircraft and private automobiles with two passengers. Maglev also has relatively low environmental impact, making less noise than any other system for the speed and requiring less land than any other mode per unit capacity. Maglev may also be as safe or safer than other modes of travel. Elevated guideways will be used so the vehicle will not encounter other vehicles, such as automobiles, at grade crossings. No on-board fuel is required, minimizing fire accidents. The vehicle does not touch the guideway, thereby minimizing wear and the effects of weather related accidents. For a more detailed discussion of the prospects for Maglev, see generally, R. D. Thornton, "Beyond Planes, Trains and Automobiles, Why the U.S. Needs a Maglev System," pp. 32-42, TECHNOLOGY REVIEW, (April 1991).
FIG. 1A shows schematically a Maglev vehicle 102 traveling along a monorail guideway 104. A first car 106 and a second car 108 are shown, however, numerous cars are possible. The guideway 104 is continuous, extending along the entire route that the vehicle will travel. The guideway may be elevated above the ground, so that the vehicle is on the order of 10 to 30 feet (3-9 m.) above the ground, or at grade, or below, depending on local conditions.
A maglev vehicle must be spaced away from its guide surface, so that there is no friction or other drag force arising from such contact. In fact, this reduced drag aspect of Maglev transportation is one of its principal attractions. Thus, one aspect of a Maglev system is the suspension apparatus that supports the vehicle at the desired vertical location for operation. The suspension maintains the vertical gap V, as desired. The suspension forces should also serve to dampen out any upward motion of the vehicle, caused by track irregularities. Thus, the vehicle moves up and down, generally along an axis V of vertical perturbation (FIG. 1B), due to the effects of gravity and track irregularities. The suspending forces arise due to the interaction between magnetic fields resulting from superconducting magnets, typically carried by the vehicle, and conductors fixed to the guideway. Typically, the power for the suspension system comes from the forward motion of the vehicle. Therefore, when the vehicle is at rest or moving slowly, alternate means of suspension are used.
Rather than using a monorail guideway, designs have been proposed where the guideway is a channel. In any case, it is typical among the various Maglev designs under consideration, for the vehicle to travel at a certain vertical distance V above the guideway, whether a monorail or a channel. This distance may be measured between the top of a monorail and the bottom of the undercarriage 110. If the train travels in a channel, such as that shown in U.S. Pat. No. 3,470,828, issued in 1969 to J. Powell and G. Danby, the distance may be measured between the bottom of the vehicle and the bottom of the channel, or from outriggers for housing superconducting magnets and some aspect of the channel wall.
If power and efficiency are not affected, it is generally more desirable to have a larger vertical gap V, rather than a smaller gap. A larger gap minimizes the danger of damage from foreign articles resting on top of the guideway. Further, a larger gap minimizes the likelihood that the vehicle will contact the guideway as a result of perturbations in the vehicle travel.
Typically, when the vehicle is at rest, some part of it, either wheels, or skids, etc., comes into contact with a portion of the guideway. Under most designs, as the vehicle begins to move forward, the suspension system begins to take effect. At that point, the wheels or skids may be retracted, as in an airplane, thereby resulting in no physical contact between the vehicle and the guideway. According to other schemes, the suspension system of the vehicle will lift it from a relatively low rest position to a relatively higher moving position. It has also been proposed to use air cushions, either generated by the vehicle or the guideway, for instance at a station, to suspend the vehicle when at rest or when, for other reasons, the electromagnetic suspension system is not operational.
Maglev depends on using very powerful magnets to provide the lift and guidance, and usually the propulsive force as well. The magnetic forces are generated by high, persistent electric currents circulating in loops through superconducting materials. The superconducting materials must be kept at very low temperatures, typically below 10.degree. K. The temperature is maintained by closed cycle cryogenic refrigeration units or in stored cryogenic fluids. With the advent of high temperature superconductors, it may be possible to operate under superconducting conditions with temperatures as high as 77.degree. K., or even higher, depending on future developments in the superconducting field. The superconducting current carrying loops are referred to in this specification as "magnets," as their principal relevance to the invention is the way in which the magnetic field from the superconducting current interacts with conductive components of the guideway.
There are two basic Maglev methods for providing upward vertical forces, known as levitation or suspension. The first is known as "attractive," or "electromagnetic suspension" or "EMS." The second is known as "repulsive," or "electrodynamic suspension" or "EDS."
EMS suspension relies principally on attractive force. EMS systems are unstable unless the current in the magnets can be varied rapidly and widely via electronic control. With non-superconducting vehicle magnet conductors, this EMS suspension requires a very narrow air gap between the guideway or guiderail and the vehicle. The practical gap for non-superconducting EMS suspension is three-eighths of an inch (1 cm.) or less, to maintain acceptable power consumption, vehicle weight and guideway cost.
In contrast, an EDS suspension system is inherently stable. The current induced in the guideway increases as the gap shrinks, thereby increasing the repulsive force, and providing steady suspension. Using EDS suspension, the magnetic field can be constant, and thus can be supplied by superconducting magnets, allowing a gap of between two to six inches (five to thirty-eight cm.).
EDS suspension systems proposed to date have drawbacks. Typically, they have been less efficient and have required more power than existing EMS suspension designs. Efficiency is defined in terms of suspension force per unit of guideway power dissipation. Another problem arises because powerful superconducting magnets produce powerful magnetic fields. It is believed that exposing humans to such magnetic fields may be harmful. Therefore, it is important to minimize such exposure.
In order to overcome the efficiency drawbacks, the conductive components of the guideway which interact with the magnetic field of the magnets can be specially designed. However, it is important to minimize the cost of the guideway, since the guideway extends for miles and miles and constitutes a significant capital cost. In fact, costs savings in the guideway are typically more valuable than cost savings in the vehicle or magnets.
It has been proposed to use a solid sheet for the guideway conductors. A relatively thick and wide sheet of conductor can be located adjacent, typically below, the conductors of a superconducting magnet, which has its main magnetic field aligned vertically. Induced guideway currents would flow in the sheet primarily under the region of the magnet conductors. However, this system is inefficient due to a skin effect phenomena. This results in a high percentage of the power delivered for suspension purposes being dissipated in the continuous sheet of the guideway without providing any lift force.
Additional problems that arise from a continuous sheet are high drag at low speeds, which can cause overheating of the guideway in low speed regions, such as near stations. The drag also requires wheels at speeds less than 30 m/s (67 mph). Wheels are undesirable for many reasons, including reliability, and the requirement that all sections of the guideway must be able to support concentrated loads that wheels create. Further, there would be little magnetic damping of vertical motion, so other shock absorbing means are required. Also, no downward suspension forces are generated, which raises instability problems. A high, induced current in the guideway induces a high reaction magnetic field and AC losses in the superconducting magnets when the vehicle operates at high speed over a guideway that is not perfectly smooth. Also, mounting problems arise due to thermal expansion.
Discrete loops can also be used, which can be designed to provide somewhat lower losses than a continuous sheet.
Ladder-like configurations have also been proposed for guideway conductors in suspension systems. A ladder is analogous to the rotor of a squirrel cage induction motor cut and rolled out flat. It is possible to reduce the cost of the guideway using a ladder of solid conductors. However, high reaction magnetic fields also typically arise in solid conductors of a ladder, resulting in the same efficiency problems as arise with a continuous sheet. Further, the efficiency is worse than with either a continuous sheet or discrete loops.
An EDS suspension design proposed in Japan uses a horizontal gap between vehicle magnets and the side wall of a U-shaped guideway channel. The bottom portion of the vehicle carries the magnets on a side face of the vehicle, which faces the side wall of the channel. A magnetic force arises as a shear force between the vertical surfaces of the vehicle magnetic coil and a series of special conductive loops in the guideway side wall. The guideway loops are in the form of figure-eights.
This is a variation of a system proposed by Danby and Powell in 1966, described in the U.S. Pat. No. 3,470,828 identified above. The Danby and Powell system was used for guiding the vehicle laterally between the sides of the channel. The coils and loops were rotated 90.degree., so that the gap was vertical. However, the principal is the same.
The Japanese EDS design is more efficient than EDS systems that use discrete loops in the guideway. However, it is more expensive to make the guideway, given the more complicated structure of the figure-eight loops. FIG. 2 shows the side wall system schematically. A portion 202 of the vehicle is shown riding within a channel. The vehicle travels in the direction of arrow F. The right (starboard) side wall 204 of the channel is shown. The left (port) side wall is not shown, for clarity. The major portion of the vehicle, including the passenger compartment, would extend above the portion 202 shown. The passenger compartment may or may not be above the maximum vertical extent of the sidewall 204.
A representative vehicle magnet 206 is formed of a loop or coils of loops of superconducting material, having a persistent current, for instance as indicated by the arrows circling its perimeter. In the full vehicle, several such magnets would be aligned along the length of each vehicle car. The persistent current generates a magnetic field having a polarity conventionally designated as north or N. A series of figure-eight loops 210 are fixed to the side wall along the entire length of the guideway. Two representative figure-eight loops 210.sub.a and 210.sub.b are shown on the left side of the guideway. The figure-eight loops supported by the right side 204 of the guideway are shown partially in phantom. The figure-eights are each conductive around their loop, so that a single conductive loop folded into two lobes, is formed. No external current or power is provided to the figure-eight loops. Current does arise in them, as described below, induced by virtue of the vehicle coils 206 moving past the figure-eight loops 210.
The means by which the Japanese EDS suspension system functions is as follows. A vertical equilibrium position for the vehicle relative to the guideway is established, in which current is induced in the guideway figure-eight loops 210 which creates a magnetic field that reacts with the magnetic field from the vehicle coils to support the vehicle at a desired height.
If the vehicle magnet coil 206 is centered between the two lobes of the figure-eight 210, the magnetic flux field of the single magnet coil 206 induces equal and opposite voltages in the two lobes of an individual figure-eight, for instance 210.sub.a. Thus, no net current results in the guideway figure-eight loops 210, if the vertical position of the vehicle coil 206 remain symmetrically located between the two lobes. (This is typically not the case.) If the vehicle falls or rises relative to the guideway along the axis of vertical perturbation, either the upper or the lower lobe of each of the figure-eight coils 210 would be more tightly coupled to the vehicle magnet 206 than the other lobe in the same figure-eight. This would induce a net unbalanced voltage between the upper and lower figure-eight lobes, thereby producing a circulating current within the figure-eight loop 210. In accordance with Lenz's law, the magnetic field arising due to the circulating current in the figure-eights 210 would interact with the field due to the vehicle coil 206, and provide a restoring force, tending to return the vehicle body 202 to the vertical equilibrium position.
Thus, if the vehicle 202 is at a vertically symmetric position relative to the guideway figure-eight, as it travels along in direction F, no current circulates in the figure-eight loop 210. This is illustrated schematically in FIG. 3A, which shows schematically in cross section a vehicle coil 206, composed of the upper and lower conductor sections 206.sub.u and 206.sub.L respectively. Current is indicated flowing in the direction shown in FIG. 3, by the "+" symbol in conductor section 206.sub.u, indicating current flowing into the page, and the "-" symbol in conductor section 206.sub.L, indicating current flowing out of the page. The conductors that make up the figure-eight, 210.sub.u, 210.sub.n, 210.sub.m, and 210.sub.L, carry no current, as indicated. (It will be understood that, typically, the vehicle is not at this vertically symmetric position, but, rather, rides below it.)
However, if the vehicle were to be located below the symmetric position, as shown schematically in FIG. 3B, the currents in the vehicle conductor would remain, but a current would be induced in the figure-eight 210, as indicated, which would generate a magnetic field that would interact with the magnetic field generated by the vehicle coil 206 to push the vehicle back upward. (It is also appropriate to regard the magnetic field as interacting directly with the current induced in the figure-eight coil.) The mutual inductance between the vehicle coil 206 and the figure-eight 210 rises very significantly with displacement from equilibrium. This rapid increase contributes to high efficiency.
Similarly, if the vehicle were to be jolted upward above the symmetric position, the current induced in the figure-eight 210 would generate a magnetic field that would oppose the relative upward motion, thereby bringing the vehicle back up toward the vertically symmetric position.
Because the vehicle has mass, it is constantly being pulled downward, away from the symmetric position, by gravity. Being away from the symmetric position, currents are induced in the guideway figure-eight loops 210, which generate a magnetic field that interacts with the vehicle magnetic field, tending to resist the gravity force, thus supporting the vehicle.
Despite the foregoing operation, the Japanese EDS suspension system is relatively costly to fabricate, align and install in the guideway the many figure-eight loops 210 that would be needed. Further, the figure-eight configuration uses relatively more metal than is desirable, for cost purposes.
There is also a need in Maglev applications to guide a vehicle transverse of the guideway (i.e., laterally, from port to starboard (left to right)). As the vehicle travels along, it may oscillate transverse of the guideway, generally along a horizontal transverse axis of perturbation T (FIG. 1B). As is mentioned above, a system has been proposed in the Danby and Powell Patent which is similar to the Japanese EDS suspension system described above. However, the guidance system is rotated by 90.degree. with respect to the vertical. When the vehicle is centered no current arises in the guideway figure-eights, which are oriented generally with the loops in horizontal planes. It is desirable to use a guidance system which uses the same vehicle magnets as are used for the suspension apparatus.
Thus, there is a need for a system to suspend a Maglev vehicle at a desired equilibrium height or to guide such a train along a guideway, in an energy efficient manner, which minimizes overall cost. Related to this objective is to facilitate the manufacture and repair of the guideway by simplifying its construction. It is also desirable to operate such a suspended Maglev vehicle at a reasonably large gap distance away from the guideway, thus ensuring that the vehicle never touches the guideway, even in worst case combinations of turn radii, vehicle speed and wind forces. Another object of the invention is to configure the superconducting magnets to minimize the magnetic field that will pass through the passenger compartment. There is also a need to suspend or guide a Maglev vehicle in such a way that permits energy efficient use of the superconducting magnets for propulsion of the vehicle and for guidance of the vehicle with respect to its lateral position relative to the guideway. Another object of the invention is to suspend a Maglev vehicle using magnetic forces, while the vehicle is traveling at slow speeds, without wheels. Yet another aspect of the invention is to use the same vehicle magnets for both the vehicle guidance apparatus and the suspension apparatus.